An isolator with appropriate harmonic filtering AFTER it (they create harmonics, which are not friendly to your neighbors), presents a nice solid non-reactive 50 ohm load to the transmitter. It doesn't matter if it's a GE MASTR II PA, or someone elses. Many Motorola PA's have the isolator built in, for example.
The GE PA likes a 50 ohm load or it tends to do squirrelly things, like die. You can get this by proper choice of QUALITY products for your filtering/feedline/antenna system, or you can use an isolator, or you can have the "belt and suspenders" mentality and always run an isolator on the output of the PA. Another good reason to do this at high RF commercial sites is that the PA (depending on how you may have modified it) has transistors in the final section that are "always on", thus... mixing can occur in the PA if RF from the outside world can get down INTO the PA from elsewhere. Also not a nice thing for your neighbors. The final "benefit" to a properly sized dummy load on your isolator is that the antenna can literally fall off the tower, and the PA will still be driving into a solid 50 ohm dummy load. As long as that load is the correct size for your PA's power level, no damage to the PA will occur, even though your antenna is gone. Some late-model MASTR II PA's have built in Z-matchers on their output Low-Pass Filter board on the right lower side. These have to be adjusted correctly per the manual any time a new PA is installed on a new antenna system, or anything changes in the antenna system. Older PA's do not have this feature, but an off-board Z-matcher can be added in lieu of an isolator if all you're concerned about is matching the source (PA) to the load (antenna system). The GE PA comes in different sizes for different power level needs. The VHF 100W (original with four final driver transistors) was known to get spurious and throw stuff up and down the band for various reasons. The newer three-final-transistor PA's (the most commonly seen) are less prone to do this, but can also become RF noisemakers if turned down too much. No MASTR II tech I know of will recommend turning any MASTR II PA any further down than roughly 2/3's its maximum rated power output, ESPECIALLY not without looking at it on a spectrum analyzer to make sure it's not spurring. When they go spurious, which is rare enough people get away with turning the PA's down more than 2/3's, they also typically overheat drastically and they'll destroy themselves relatively quickly. If you need less power than 50W it's highly recommended to remove stages of the PA to get to the lower power level you need, bypassing them completely, or moving to a lower power/lower-duty-cycle PA in the model line. Done right, even after 20 years of commercial service, the VHF MASTR II PA will happily sit there and put out 85W all day long, without batting an eyelash. Many will still do a strong 110W and never care, either. It's just a little "easier" on the gear to run the power back just a little bit. The difference (in dB) between 85W and 110W often isn't significant, depending on your area you're wanting to cover. Why are you wanting to turn your MASTR II back so far? After duplexer losses, you're down to roughly 20W if you're putting 45W into a duplexer (if you're running split antenna, with no duplexer and vertical separation, nevermind -- but I doubt you are), and 20W doesn't "balance" well with 50W mobiles. If you're trying to balance for HT coverage only, I could see it maybe... My personal rule for the MASTR II PA's is: Set 'em to 85W and forget 'em. Put an isolator on for ALL of the good reasons for an isolator, filter it properly, and forget about it. The vast majority of them will run for 10 or more years without touching them again after doing that. Look carefully at the Repeater-Builder company specifications -- you may be looking at a MASTR II *MOBILE* rig converted into a repeater. That's NOT a MASTR II Repeater PA. They're de-rating the PA to save it from overheating and dying, as it's not rated for 100% continuous duty at full power in the mobile rig with the small heatsink. The MASTR II REPEATER PA is a giant heavy thing with plenty of heatsink fins, 19" wide and multiple RU tall, that is EIA rated to do continuous-duty cycle at 110W for at least 24 hours. Nate WY0X -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Randy Sent: Sunday, February 01, 2009 8:24 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] GE Master II Pa decks Hi group whats the deal with the GE Master II PA decks needing to use a ferrite isolator with a 50 ohm dummy load and a low pass filter between the PA and the High Q duplexer. Is this something that most do with the 75 and 100 w Pa's We just replaced one in a Master II Mobile radio which lasted for years with no issues. After installing the PA and doing a tune up the Radio did about 80 watts we set it to 45 watts I was also told that was not a good thing to do so whats the story on the PA. I seen where Repeater Builder the Company talks about the 75 or 100 watt PA's in there web site only bening good set to 45 watts or so anyway so I assume they are setting theres at that level when they do a Duplex conversion ect making the radio into a repeater. I have read the information on the website about this but would like some user comments as well. Also where would one get the needed parts ect to do this Mod. Randy ------------------------------------ Yahoo! Groups Links
